We have grouped HVAC system noises into groups by noise type and some alphabetic order. We start here with HVAC NOISE group 1, banging, bearings, buzzing sounds. The article includes a complete catalog of all HVAC system noises.

This air conditioning repair article discusses the
diagnosis and repair of air conditioning compressor noises which range in importance from normal (if annoying squeaks and squeals,
to rattling loose bolts and hardware, to costly compressor damage indicating
air conditioning compressor or A/C compressors at or near end of their life.

Squealing, clanking compressor noises

This brief video contributed by one of our readers (L.M.) demonstrates start-up troubles and loud continuous squealing, clanking, and what I take to be some humming at the outdoor fan-compressor unit. The unit is unable to start and run normally.

A/C Compressor Noise Description:

Twice in the last month in a half, we’ve had this squealing metallic noise coming from the exterior condenser. On both occasions the AC was running fine all day, then about 930 or 10 PM it starts making this noise. In both cases it eventually stopped making the noises on its own. It makes the noise only while AC is running. Fan only is OK.

HVAC Noise Group 1

Reader Question: banging sounds from the compressor

OK. I just reread the instructions and it looks like I can ask a question. In the past 5 years we have replaced 4 compressors. We just replaced the last one and the fan motor. Every once in a while, the unit would make a repeated really loud banging sound as though some metal piece in the fan was whacking another piece over and over. Today I watched the clock

It is almost at every hour, but not at the same time. Each time the noise lasts 3-4 minutes. The A/C company and their electrician have no clue what this is. Also, the unit often trips the circuit breaker. Not necessarily when this noise comes on. We are incredibly frustrated. Any ideas as what could be causing this? Thank you. - Dina

Reply: common sources of loud BANG noises in air conditioning or heat pump equipment

Dina, my OPINION is that if you are replacing compressors 4 times in 5 years something is drastically wrong. I can't guess whether the problem is an innate system design pressure, poor or damaged equipment being installed, or installation errors.

But I share your frustration. I think it's time to talk candidly (but without threats or screaming) with the A/C company's service manager and to ask that they send someone who is trained and experienced to help diagnose this problem. And when you're told what problem was diagnosed, if you don't understand the explanation, ask for more help with understanding it until it makes sense.

A fan whacking something is a loose fan blade or broken fan shaft or bent screening around the fan or a stick or junk in the unit - any of which is a serious problem for the fan. If your service tech left the system like that s/he did not do such a hot job.

A unit that trips the circuit breaker is probably drawing high current - which, if the proper breaker has been installed, points to a problem with the wiring or more often the compressor motor on that circuit. A failing compressor motor draws high amps.

I'd also suggest checking out the voltage being provided to the system to be sure that there is not a low-voltage problem from your power source.

Follow-Up:

Thank you so much. Here's the problem. We just changed service companies , because we were not satisfied that the old one was doing a good job. This one decided that the compressor and then the fan needed replacing. They've had two sets of technicians out here and neither could figure out the problem. There is nothing hitting the fan (so they say). The problem is that they are never here when the sound happens, though

I did record the sound for them. They said it was nothing like anything they've heard before. They are coming out this morning again and we will see what they can find. Thank you for your "opinion". - Dina

The tech was here again. He noticed that the last guy had installed a booster. He disconnected it to see if it helps. Meanwhile, I'm wondering if the noise has something to do with the unit working harder in the hot weather because I hadn't noticed the noise at all today and it's much more comfortable outside. I guess I won't know until it turns really hot again. - Dina

Even with the booster disconnected, it's still making that banging metal sound. - Dina

Question: banging noises from heat pump when heat is running

Ron said:
My Heat Pump makes a constant banging sound only when the Heat is running. The video shows the fan moving oddly... but to the naked eye it seems fine. Any ideas?

Here is a video of the "banging" hard-starting fan on this outdoor compressor/condenser unit.

Reply:

Ron

Ordinarily I'd say that to find the cause of ticking, rattling or banging at the fan component of a compressor/condenser you want to look for a loose fan bearing or bent blade or something obstructing the fan.

But after checking the video found in your banging compressor noise video (above) I see that the motor is having trouble starting and is cycling at start-up.

Possibly the the fan has a bad start/run capacitor. Try replacing that and let me know what happens.

Banging or Hard Starting Compressor Condenser Unit Video

Bearing Noises in HVAC equipment

Reader Question:

Sounds like very loud bering noise on start up and shut down from a 1 year old heat pump compressor - Ray 12/5/11

Reply:

Ray, a bearing noise, often a horrible clanking, banging, or rattling, is generated by a wobbling of a rotating shaft that is no longer evenly supported by bearings that are intended to permit it to rotate with minimal friction.

If you hear bearing noises from an A/C compressor motor I suspect that the motor is failing, or that a similar noise is being made by damaged refrigerant valves in the compressor head. But first check that the motor mounts are secure. Sometimes a motor can make a clank or thunk at startup because one or more of its mounts has come loose.

Reader Question: loud buzzing noise from compressor unit

I had problems with my central unit freezing up and dripping water on my fan motor.. decided to have a technician change out my coil inside.. when he left my central unit outside starting making noises i never heard before.. my air conditioner kicks on inside first then after a couple of minutes it tries to kick on outside , it makes a loud noise then it runs finally and quiet like a central unit should run outside.. what is causing this all of a sudden, never had problems until he changed my coils out..not calling him back to my house. Bruce McCann

Reply:

HVAC compressor buzzing video: the HVAC noise diagnosis introduction near the top of this page includes a video example of brief buzzing at the fan/compressor outdoor unit along with some possible explanations. Below we discuss several A/C or heat pump compressor noise observations and possible explanations.

Bruce, I am not clear on why a coil freeze-up indoors would lead to changing the outside coil - maybe low refrigerant or improper high side vs low side pressures? In any case, I would indeed give the service tech a call, tell him or her what you are observing, and give the technician an opportunity to return, see what's wrong, and fix it.

Most technicians want their work to be correct and successful. Let us know what you're told - it will surely help other readers.

Question: can you tell us what the attached air conditioner buzzing clanking sound recording means?

Attached is a few seconds of the noise made by our Kenmore air conditioner/heat pump, when the propellers start turning, about 10 seconds into the recording.
It was much worse in the winter with the heat pump, so we heated with oil only.

It was so bad, the house rattled inside. The compressor and motor were replaced a few months ago, but the noise persisted and the maintenance company gave up after eight visits.
When the weather got hot, we decided to give the air conditioner a try.

The air conditioner was working without us noticing any noise but now we can hear it, as you can tell from the recording.
What can we do? Your help would be greatly appreciated. - L.G.

Reply:

Honestly, I'm not sure , but the sound we heard in your recording sounded like a mix of buzzing and rattling. I've heard sounds similar to this from both a failing crankshaft in a motor and from an air conditioner compressor that was liquid slugged with refrigerant due either to an improper charge or a malfunctioning refrigerant metering device.

If that's right we may be hearing a hard-starting compressor.

I have two suggestions: a technician, using a mechanic's stethoscope, ought to be able to pinpoint this noise to the offending part or motor. If the noise is traced to the compressor motor I suspect my guess above is not far off. It is possible that once the system has been running for enough time the noise may fade as the compressor has pushed the refrigerant out to the high side of the system.

Watch out: liquid refrigerant slugging of a compressor motor is a problem that occurs on the low pressure side of the system where the equipment is designed only to move refrigerant as a gas, not a liquid - liquid slugging of refrigerant at the compressor motor is likely to destroy it.

I hope you've already eliminated more obvious external rattles such as a bad bearing or motor shaft or fan blade ticking against screen or internal component.

At Recordings of A/C or heat pump sounds & noises we have introduced a new HVAC diagnostic feature (thanks to you) where we posted this air conditioner sound recording to permit other readers or experts to offer comments.

Question: Buzzing sounds from air conditioner

My outside fan motor is running but the unit makes a loud buzzing noise every few minutes or so, the motor was replaced last spring. I sprayed the motor shaft with wd40 just to get me by till I can get another fan motor if that is needed. I do not know if the capacitor was replaced or not. the loud buzz only lasts for a few seconds then stops, I just turned the unit back on, Any ideas on what I can do or a place to find a motor?- Rodney Looney

Compressor started producing a buzzer noise which lasted about 4 seconds then stopped and repeated again. The time between buzzes varied from a 10 to 20 seconds. The noise reminded me of the sound you hear on the radio when they do there check of the emergency broadcast system. - Jim

I have a Trane 4 ton gas pack that is making a intermittent buzzing noise when the compressor operates. We are located in Phoenix, AZ and the noise seems to have started this season when temps heated to 100 and above. Naturally, I'm concerned that this might be compressor noise since the unit was manufactured and installed before the AC season in 1994. Thanks for any suggestions you can offer. Doug

More info:
The unit is on the roof of a flat roof house and the noise is most noticeable when standing below the unit. The noise is a higher frequency buzzing that could be a panel or other object vibrating close to the frequency of the compressor. To my knowledge, I have not been on the roof when the noise occurred, or if it did, I didn't hear it with all the other noise. It seems to occur at various combinations of temperatures.

Under some conditions, there is no noise, but other times, such as during a long run when we lower the temp after the sun sets, the noise may start several minutes into the cycle. A few times, I noticed noise soon after starting that only lasted for 20 seconds or so. We run the indoor temps between 85+ and 81 over a 24 hr period using a programmable thermostat.

I have banged and thumped on the case, removed the outdoor fan grill and looked for loose fasteners within the compressor/condenser portion of the case and checked all of the fasteners on the outside of the case to make sure they were tight. I put card-stock shims between joints of panels, etc to prevent any vibration that might occur. So far, nothing has eliminated the noise that seems to occur only under certain combinations of outdoor/indoor conditions.

The noise pattern may be different from when I first noticed it. I checked the pressures, for the first time since we got the house 8 years ago and and added 4 lbs of R-22 to get it back to normal operating conditions at 95F. The performance isn't noticeably different, but the noise pattern may have changed somewhat, while the sound is about the same, when it does occur.

Last winter I removed the top of the unit to oil the indoor fan motor. That is the first time it has been removed since we owned the house. As far as I know, everything was reassembled correctly. My experience is limited to light to medium maintenance of only my own auto/home AC systems for the past 4 decades. I installed a quick start system about 5 years ago on general principals because one worked wonders on similar but smaller gaspack in another part of the house. - Doug

Reply: Buzzing Sounds at an A/C or Heat Pump System May Mean Failing Electrical Parts

Rodney: if you need a new condenser fan motor you should be able to match the original by taking the old one to your local HVAC supplier, or you can probably find a replacement online. I'd be sure you note the motor information and specs from it's data tags or just bring in the old motor to your supplier. Watch out - don't keep replacing motors unnecessarily. A bad fan motor contactor relay or even a loose wire could also cause buzzing in the condenser/compressor unit outdoors. .

Jim: sounds like your compressor is having trouble starting up? Or does it buzz but run ok otherwise?

Doug,

buzzing sounds from an air conditioning component might be a failing electrical component and could involve even a fire risk; I'd try to track down more carefully where the buzzing is occurring. A burned compressor contactor relay, for example, might buzz.

Noise diagnosis is tricky in part because there is some subjectivity in how people describe sounds (buzzing vs humming for example). Taking care to not get whacked by a moving part like a compressor fan, one can sometimes use a mechanic's stethoscope to get right to the source of a sound from mechanical equipment.

Follow up:

Dan,
Thanks for your suggestions. I'm relieved that you didn't point to the compressor as a typical source for noise like mine, but I know I can't rule it out yet. I took a dowel rod up to listen to for any noises, but at 94 deg, it didn't make any abnormal sounds (my wife confirms). I did notice that the top of the condensor at one edge was vibrating and a potential source of noise. I'll check it later. I opened the control compartment and all seemed normal and no loose components there. I would like to be able to listen to the compressor, but there's no way without dueling with the outdoor fan and I would likely lose. I'll let you know if I ever come to any conclusions.

Follow up:

Dan,
A while ago, I wrote about intermittent noise from my roof mounted gas pack. After spending a lot of time on the roof and poking around inside the compressor compartment, I am reasonably certain the noise is coming from the compressor itself. I checked the compressor mounts and they are solid and there are no loose parts to vibrate and cause noise. In the process of pushing the compressor around to see if the mounts were loose, I heard the compressor hit the side of the shell.

I was looking around another site and the owner mentioned that the compressor suspension mechanism can break and cause noise that sounds like a bad bearing.

A friend of mine knows a person in the refrigeration business and after my friend described the problem, he said it sounded like a broken spring in the compressor suspension.

I guess I am going to have to replace the compressor before the start of the next season, if I make it through this one.
Hopefully I will replace it before there is a burnout. Would you recommend installing a filter/dryer even if there is no hard failure? I don't believe Trane installs them in new-build units and there is certainly none in the compressor bay. If the compressor banging against the shell is releasing particles into the system, is there a filter or something other than a dryer that will catch the particles?

The compressor has rotolock connections, and I would like to avoid any cutting/sweating operation if I can reasonably-safely avoid it. Thanks for your help. - Doug

Reply:

Doug:
re: noise is coming from the compressor itself - a failing compressor motor indeed can become noisy;
I would not install a filter/dryer before replacing the compressor because
- an extra cycle of opening the refrigerant piping system risks contamination and also done properly requires vacuuming and cleaning the whole system - it's a bit of a costly procedure to do at that point
- when the compressor is replaced a filter/dryer would normally be installed as part of the job

But if you wait for a failing compressor to burn up the result can be extra contamination in the refrigeration system.

For those reasons I'd ask for an experienced HVAC tech to look at the system, diagnose the problem, and then I'd decide if it were time and better to replace the unit now, or shut the system down and replace it before start of next season.

Follow-Up:

Dan,
Thanks for the tips. All makes sense to me. I will consult with the a dealer that I know, before I do anything. I am inclined to just replace the compressor though, if I can find one at a reasonable cost. I found a new AM Std compressor on eBay that appears to be interchangeable with the one in my unit, but so far, I can't find cross reference data that I trust. I will probably use it lightly until we get into cooler weather (6 weeks)and hope for the best, but prepared to replace the whole unit if crashes. I'll add a filter/dryer if I replace the compressor. Sorry if I didn't make it clear, I wasn't thinking of a two step operation.

I found a video on You Tube that shows disassembly of a compressor like mine. I don't think there is a broken suspension, but I would believe bearing noise. I don't understand how it can be so intermittent. Under certain conditions, something resonates and makes a real racket.
Thanks again for the help. - Doug

Dan,
Regarding installation of a filter/dryer; the filters I have seen on HVAC parts sites are for installation on the suction side. When I installed an AC in my house in '88, I installed a filter/dryer in the liquid line, near the coil as recommended at the time. It seems to me that it makes more sense to filter the liquid entering the TXV than to filter the gas entering the compressor. What am I missing? I purchased the compressor on eBay. It turns out that it was exactly the P/N specified in the parts list for the equipment. The gaspack contained what I assume was an earlier equivalent part. - Doug

Reply:

Doug we see filter dryers installed at both sides of the compressor. It depends ... if a system is thought to be contaminated there are advantages of having a filter at both ends of the compressor motor, and it certainly doesn't hurt. - DF

Outdoor unit makes a buzzing noise for about 2 seconds & stops. Fan never starts turning. I was standing by unit when this happened for the third time & manually started fan turning.
Fan started running on its on, but at a very low speed, nothing like it normally does. Any suggestions? Inside unit seems to work normally. - Hill

We seem to have traced this A/C buzzing noise to a bad start/run capacitor - Hill

Continue Reading more Noise Descriptions at noise group 2 beginning with CLANKS CLUNKS from air conditioner or heat pump system

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Technical Reviewers & References

Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.

Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.

Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

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Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.